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Somatosensation
Touch, Pain, Temperature

Explore how your body detects pressure, vibration, temperature, and pain through specialized receptors in the skin and deeper tissues.

Anatomy & Physiology

What Is Somatosensation?

Somatosensation is the collection of sensory modalities that allow you to perceive touch, pressure, vibration, temperature, pain, and body position. Unlike vision or hearing, these receptors are distributed throughout the entire body: skin, muscles, joints, and internal organs.

The Four Modalities

Mechanoreception
Detects pressure, vibration, stretch, and texture through mechanical deformation of tissue.
Nociception
Detects tissue damage or potentially harmful stimuli, perceived as pain.
Thermoreception
Detects changes in temperature: warmth receptors and cold receptors.
Proprioception
Detects body position, movement, and muscle tension via receptors in joints and muscles.

Why Somatosensation Matters

Without somatosensory input, you could not withdraw your hand from a hot stove, feel the texture of fabric, maintain balance while walking, or detect an injury. This system is your body's first line of defense and a constant source of environmental information.

1
Survival: pain signals trigger reflexive withdrawal from danger.
2
Coordination: proprioception enables smooth, coordinated movement.
3
Social bonding: gentle touch triggers oxytocin release and emotional comfort.

Receptors in the Skin

EPIDERMIS DERMIS HYPODERMIS Merkel Light touch Meissner Texture Ruffini Stretch Pacinian Vibration Free nerve Pain/Temp

The Sensory Pathway

Receptor Sensory Neuron Spinal Cord / Brainstem Somatosensory Cortex Transduction Transmission Perception

DCML vs. Spinothalamic Tract

DCML Pathway Spinothalamic Tract
Carries Fine touch, vibration, proprioception Pain, temperature, crude touch
Fiber type Large, myelinated (A-beta) Small, thinly myelinated / unmyelinated (A-delta, C)
Crosses midline In the medulla In the spinal cord
Speed Fast Slower
Relay Thalamus (VPL nucleus) Thalamus (VPL nucleus)

Receptor Adaptation

Sensory adaptation is the decrease in receptor sensitivity during prolonged, constant stimulation. You stop "feeling" the watch on your wrist because those receptors adapt rapidly.

Rapidly Adapting
Respond at onset/offset of stimulus. Detect changes and movement. Examples: Meissner and Pacinian corpuscles.
Slowly Adapting
Continue firing as long as stimulus is present. Detect sustained pressure. Examples: Merkel discs and Ruffini endings.

Pain: The Nociceptive System

A-delta Fibers Fast, sharp pain "First pain" : localized C Fibers Slow, burning pain "Second pain" : diffuse Conscious Pain Somatosensory Cortex

Gate Control Theory

Proposed by Melzack and Wall (1965), the gate control theory explains why rubbing an injured area reduces pain. A "gate" in the spinal cord dorsal horn can be opened or closed to modulate pain signals before they reach the brain.

1
Gate opens: small C fibers (pain) activate transmission neurons, sending pain signals upward.
2
Gate closes: large A-beta fibers (touch) activate inhibitory interneurons that block the pain signal.
3
Brain influence: descending signals (endorphins, attention, mood) also modulate the gate from above.

Temperature Sensation

\(10^\circ\text{C}\)
\(20^\circ\text{C}\)
\(30^\circ\text{C}\)
\(40^\circ\text{C}\)
\(50^\circ\text{C}\)
 Cold receptors Warm receptors Firing Rate Pain (cold) Pain (heat)

Receptive Fields & Acuity

Fingertip Small field
\(\sim 2\,\text{mm apart}\)
 Back Large field
\(\sim 40\,\text{mm apart}\)
 vs.

The Somatosensory Homunculus

The primary somatosensory cortex (S1) is located on the postcentral gyrus of the parietal lobe. It is organized as a somatotopic map: each body region has a dedicated cortical area. The more sensitive the body part, the larger its cortical representation.

Large Cortical Area
Hands, lips, tongue, face: regions with high receptor density and fine discrimination.
Small Cortical Area
Trunk, back, legs: regions with lower receptor density and less spatial acuity.

Referred Pain Explained

Heart (visceral pain) Left Arm / Chest (perceived location) Shared Neuron Brain

How to Classify a Receptor

1
Identify the stimulus type: is it mechanical pressure, temperature change, tissue damage, or chemical?
2
Determine the location: is the receptor in the epidermis (superficial) or dermis/hypodermis (deep)?
3
Check encapsulation: is it encapsulated (e.g., Pacinian) or a free nerve ending?
4
Assess adaptation rate: does it adapt rapidly (movement detection) or slowly (sustained pressure)?
5
Identify receptive field size: small field = high acuity, large field = low acuity.

Common Mistakes

✕ Confusing adaptation types
Rapidly adapting receptors detect change, not sustained stimuli. Meissner detects texture changes, not constant pressure.
✕ Mixing up pathways
The DCML pathway carries fine touch (not pain). The spinothalamic tract carries pain and temperature (not fine touch).
✕ Assuming nociceptors adapt
Unlike touch receptors, nociceptors do not adapt to sustained stimuli. Pain persists as long as the tissue is damaged.

Receptor Summary

Receptor Stimulus Adaptation Location
Merkel disc Light touch, edges Slow Epidermis
Meissner Texture, flutter Rapid Dermis (superficial)
Pacinian Vibration, deep pressure Rapid Hypodermis
Ruffini Skin stretch Slow Dermis (deep)
Free nerve endings Pain, temperature Non-adapting (pain) Epidermis

Question 1 of 5

Which receptor is most responsible for detecting vibration deep in the skin?

Question 2 of 5

A patient has damage to the spinothalamic tract. Which sensation would be most affected?

Question 3 of 5

Why does rubbing an injured area reduce pain, according to the gate control theory?

Question 4 of 5

A person can distinguish two points \(\sim 2\,\text{mm}\) apart on their fingertip but requires \(\sim 40\,\text{mm}\) on their back. This difference is best explained by:

Question 5 of 5

A patient experiencing a heart attack reports pain in the left arm and jaw rather than the chest. This phenomenon is called: